You are right to be concerned. If your son's reading subtest scores are below average in first grade, he has a problem.
Reading problems are not caused by immaturity and cannot be outgrown. The notion that children who are late bloomers in reading will catch up when they get older is a myth.
If your son has a reading problem in first grade, he will have trouble when he reaches third or fourth grade. Rather than waiting for your child to "mature," he needs to be taught to read using the rules of phonics, the tools he needs to recognize new words.
You need to get a comprehensive reading assessment.
Tests that measure your child's phonological processing, phonics skills, and spelling will clarify why your child is having difficulty and what skills need to be addressed.
For a complete listing, see Table 6-1 Reading Tests and the Skills They Measure in Wrightslaw: All About Tests and Assessments, 2nd Edition, page 60. For more information about Reading Skills see page 56.